The present invention relates to a method and a device for preprocessing the acoustic signal delivered to a speech coder. It applies especially, but not exclusively, to improving the performance of low bit rate speech coders.
The present-day speech coders with low bit rate (typically 5 kbit/s for a sampling frequency of 8 kHz) yield their best performance on signals exhibiting a "telephone" spectrum, that is to say one in the 300-3400 Hz band and with pre-emphasis in the high frequencies. These spectral characteristics correspond to the IRS (Intermediate Reference System) template defined by the CCITT in Recommendation P48. This template has been defined for telephone handsets, both for input (microphone) and output (ear pieces).
However, it happens more and more frequently that the input signal of a speech coder exhibits a "flatter" spectrum, for example when a hands-free installation is used, employing a microphone with linear frequency response. Conventional vocoders are designed to be independent of the input with which they operate, and, besides, they are not informed of the characteristics of this input. If microphones with different characteristics are likely to be connected up to the vocoder, or more generally if the vocoder is likely to receive acoustic signals exhibiting different spectral characteristics, there are cases in which the vocoder is used in a sub-optimal manner.
In this context, a main purpose of the present invention is to improve a vocoder's performance by rendering it less dependent on the spectral characteristics of the input signal.